A monolithically integrated multi-transducer microsystem to detect organic and inorganic gases is presented. The system comprises two polymer-based sensor arrays based on capacitive and gravimetric transducers, a metal-oxide-based sensor array, the respective driving and signal processing electronics and a digital communication interface (see the first figure). The chip has been fabricated in industrial 0.8-μm, complementary-metal-oxide-semiconductor (CMOS) technology with subsequent post-CMOS micromachining. The simultaneous detection of organic and inorganic target analytes with the single chip multi-transducer system has been demonstrated. The system is very flexible and can provide different information of interest: the capacitive sensors can, e.g., act as humidity sensors to deal with the cross-sensitivity of the metal-oxide-based sensors to water, or the capacitive sensors can be coated with differently thick polymer layers to detect organic volatiles even in a background of water. The multi-transducer approach provides a wealth of information that can be used to improve the system discrimination capability and performance in gas detection.

We report on results achieved with three different chemical microsensor systems featuring three different types of transducers, all of which are monolithically integrated with associated driving and readout circuitry. The capacitive sensor which is sensitive to changes in dielectric properties of the polymer layer upon analyte absorption, and the cantilever which is sensitive to predominantly mass changes, are two polymer-based gas sensors. An on-chip integrated /spl Sigma//spl Delta/-converter is used to detect the minute capacitance changes of the capacitive sensor. The cantilever is magnetically excited and its vibration is detected using a piezoresistive Wheatstone bridge. Self-oscillation of the cantilever is achieved through monolithic integration of the feedback circuitry. The third transducer is a microhotplate-based gas sensor which relies on the resistance changes of a metal oxide upon gas exposure at an operation temperature of 300-400/spl deg/C. The ultimate goal is the monolithic integration of all the different transducers with driving and signal-processing electronics and a digital communication interface on the same single chip.

We present a novel single-chip smart chemical microsensor system fabricated using industrial CMOS technology and post-CMOS micro machining. It combines three different micromachined transducers (mass-sensitive, capacitive, and calorimetric) all of which rely on polymeric coatings as sensitive layers to detect airborne volatile organic compounds (VOC). A temperature sensor is included to account for the strong temperature dependence of volatile absorption in polymers. Integration of microelectronics and micromechanical components on the same chip allows for controlling of the sensor functions, and enables on-chip signal conditioning that drastically improves the sensor performance. The circuitry includes biasing, amplification, and a serial interface to transmit data to off-chip recording units. The chip forms an integral part of a handheld chemical sensor unit to discriminate and quantify VOC's.

Research activity in chemical gas sensing is currently directed towards the search for highly selective (bio)chemical layer materials, and to the design of arrays consisting of different partially selective sensors that permit subsequent pattern recognition and multi-component analysis 1 , 2 , 3 . Simultaneous use of various transduction platforms has been demonstrated 4 , 5 , 6 , and the rapid development of integrated-circuit technology has facilitated the fabrication of planar chemical sensors 7 , 8 and sensors based on three-dimensional microelectromechanical systems 9 , 10 . Complementary metal-oxide silicon processes have previously been used to develop gas sensors based on metal oxides 11 and acoustic-wave-based sensor devices 12 . Here we combine several of these developments to fabricate a smart single-chip chemical microsensor system that incorporates three different transducers (mass-sensitive, capacitive and calorimetric), all of which rely on sensitive polymeric layers to detect airborne volatile organic compounds. Full integration of the microelectronic and micromechanical components on one chip permits control and monitoring of the sensor functions, and enables on-chip signal amplification and conditioning that notably improves the overall sensor performance. The circuitry also includes analog-to-digital converters, and an on-chip interface to transmit the data to off-chip recording units. We expect that our approach will provide a basis for the further development and optimization of gas microsystems.

We present a new method for discriminating organic vapors based on the variation of capacitance changes of an interdigitated CMOS capacitor with the thickness of the sensitive polymer layer. By carefully adjusting the thickness of the polymer layer, discrimination potential in addition to the chemical selectivity of the polymer is provided by the fact that the interdigitated capacitor signals depend on the layer thickness. At polymer thicknesses small compared to the center-to-center spacing of the electrodes, an increase in capacitance is observed for all analytes, whereas for thick layers,the direction of the capacitance changes depends on the dielectric constant of the analyte. Sensors can be designed to be insensitive towards a certain analyte by varying the polymer thickness. Measurements for volatile organic compounds (VOCs) using CMOS capacitors coated with different polymer thicknesses are presented to demonstrate the new way of increasing sensor selectivity.